The present invention relates to a self-scanning type photoelectric conversion element drive device which is used to detect the correct focusing position of a photographing lens employing a detected light intensity distribution such as the variation of contrast of an image.
A self-scanning type photoelectric conversion element drive device has been known in the art in which a self-scanning type photoelectric conversion element including a pair of photoelectric element arrays, each composed of a plurality of individual photoelectric detecting elements, is arranged at a position equivalent to the photographing plane of a photographing lens, and the photographing element arrays are scanned to obtain contrast values of the image at positions in front of and behind the photographing plane, thereby to detect the position at which the photographing lens is correctly focused. The contrast values of the image provided by the photoelectric elements has a distribution having a single peak at the focused position. The symmetry of the contrast distribution as reflected in the outputs of the photoelectric element arrays is utilized to detect the focused position.
If the charge accumulation time of the photoelectric elements is changed in inverse proportion to the average light intensity of the image, the photoelectric element arrays provide a constant output at all times. Accordingly, by varying the charge accumulation time, the range of effective dynamic range of the photoelectric element arrays can be made very broad. The charge accumulation time can be changed by varying the scanning frequency of the photoelectric element arrays. However, the scanning frequency is limited by the speed of operation of a contrast detecting circuit which processes the outputs of the photoelectric element arrays.
If the photoelectric element arrays are scanned in parallel (at the same time), the effective dynamic range of the device can be compared with what can be obtained using serial scanning. Accordingly, where only the dynamic range is taken into acount, parallel drive is advantageous over serial drive. However, in order to carry out parallel driving, it is necessary to provide a contrast detecting circuit of intricate construction for each photoelectric element array. Accordingly, the overall circuit arrangement of the device becomes complex and the manufacturing cost is increased.
As is apparent from the above description, the conventional self-scanning type photoelectric conversion element drive device involves two apparently conflicting requirements: an increase in the dynamic range and a simplification of the circuit arrangement. It is thus a primary object of the present invention to provide a self-scanning type photoelectric conversion drive device in which these two seemingly conflicting requirements are met.